Flow control chokes have been used for decades in drilling and production operations involving hydrocarbon recovery. Flow control chokes are commonly used on wellheads and in production manifolds to regulate the flow of fluids from a well, and to desirably reduce a high fluid pressure to a more manageable fluid pressure downstream from the choke. Chokes are also commonly used in well testing, and in stimulation and remedial recovery operations.
Most flow control chokes are either of the type which utilize multiple orifice valves (MOV) or a needle and seat arrangement both to reduce downstream pressure and to close off the flow of fluids through the choke. MOV chokes typically rotate one disk with throughports relative to a stationary disk with similar ports to control fluid flow through the choke. These chokes generally rely upon the high upstream fluid pressure to force one disk into sealing engagement with the other disk to close off flow through the choke. Chokes with needle and seat arrangements are generally considered satisfactory for operations involving relatively clean fluids, but are not widely used in applications where the fluids are highly corrosive or contain contaminants.
Many commercially available chokes rely upon a choke body design wherein the fluid inlet is angled at 90.degree. relative to the fluid outlet. While these right angle chokes are satisfactory for some applications, a choke with a fluid inlet "in-line" with the fluid outlet is preferred for various applications, such as those involving horizontal wellheads. Some in-line chokes require that the valve body be removed from the flow line to service the valve. These chokes thus experience high maintenance and repair costs, and are not generally utilized in applications involving corrosive or contaminated fluids.
Top entry chokes offer the advantage of an in-line choke design and the capability of servicing the choke without removing the body from the flow line. U.S. Pat. No. 5,217,046 discloses a top entry choke including opposing internal orifices which cause the high pressure fluid to impinge upon itself within the body of the choke. This flow impingement desirably dissipates some of the high pressure energy in the upstream fluid, thereby reducing the wear on internal choke components. The choke disclosed in the '046 patent utilizes a seal between multiple orifice disks to seal off fluid flow through the choke. The CFB choke sold by Valve Concepts International also utilizes a flow impingement design, with an outer flow cone being axially movable relative to an inner seat nozzle to control the number of flow ports exposed to high pressure fluid and thereby reduce the downstream fluid pressure to a desired valve.
Most chokes do not maintain a reliable shutoff seal over a reasonable period of time. High pressure forces acting on the MOV disks create high friction which both wears the seals and requires a high torque to operate the choke. The needle and seat which combine to form the primary seal are each in the flow path of the fluid transmitted through the choke. The seal in needle and seat chokes thus experiences high wear, particularly when the choke is used with corrosive or contaminated fluids, and when the choke operates in a partially closed position. Since complete shutoff of chokes often cannot be maintained, many applications utilize a choke to reduce downstream pressure in a flow line, and utilize a conventional gate valve downstream from the choke to reliably seal off the fluid flow through the line.
Many choke designs include numerous components which experience high wear. Accordingly, the cost of maintaining chokes is a significant factor which has limited the use of chokes. Other choke designs are complex, and cannot be easily adapted to accommodate a top entry, in-line choke design. A choke which can reliably withstand high pressure fluids and which is capable of shutting off fluid flow over an extended period of time has long been desired. Such an improved choke is capable of use in numerous applications which heretofore have utilized a choke and a separate flow control valve downstream from the choke.
The disadvantages of the prior art are overcome by the present invention, and an improved choke is hereinafter disclosed which is relatively simple in design and construction, is readily serviceable, and which is able to maintain a reliable shutoff seal to close off fluid flow through the choke. The primary seal in the choke is not in the direct flow path of the fluid transmitted through the choke, which substantially enhances the life of the choke.